LAY ABSTRACT A small but significant percentage of children are born with severe life threatening diarrhea that can be classified as congenital. These children frequently have one of a growing group of genetic disorders that generally result in improper movement of various nutrients and salts across the intestine. We have recently discovered that a group of children who are incapable to absorbing all forms of nutrients have a genetic disorder that results in the inability to form enteroendocrine cells. Enteroendocrine cells are the hormone producing cells that reside in the gut and their formation is driven by a gene named Neurogenin3 that is defective in these children. In this grant application, we propose to identify more children with defects in the NEUROGENIN-3 gene so that we can get a better understanding of the clinical implications of this disorder. We also have been working on mice that carry a similar mutation of the NEUROGENIN-3 gene and we're using it to isolate the early and late forms of enteroendocrine cells in mice and humans. One main goal is to isolate these cells is so that we can get a better understanding of how they work, and how subsets of these cells develop. We have also found that NEUROGENIN-3 causes enteroendocrine cells that are grown in the laboratory to stop dividing and to take on the characteristics of mature enteroendocrine cells. We hope to understand the mechanism of how NEUROGENIN-3 causes the cells to stop dividing and to determine if a similar process is occurring in live mice. Finally, we have evidence that a loss of enteroendocrine cells results in a dysfunction of the intestinal cells called enterocytes that are responsible for absorbing nutrients. Here we plan a series of experiments to begin understanding the mechanism of how an absence of enteroendocrine cells leads to improper handling of nutrients by the enterocytes.